A phenomenological theory of the developing of thermally fixed holograms in photorefractive ferroelectrics has been developed. The possible combinations of the two macroscopic origins of developing - spatial modulation of the photovoltaic current and spatial modulation of the photoconductivity - were considered. It was shown that the experiments on the hologram developing under the short-circuit and the open-circuit conditions prove extremely useful in choosing one of the above mentioned variants.
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